Arylene sulfide resins, ranging in consistency from viscous liquids to crystalline solids, are known in the art. While such polymers exhibit desirable properties, the unmodified, uncured, moderately branched to linear (i.e., virgin) arylene sulfide resins normally have associated therewith relatively high melt flow values (e.g., above about 4,000 grams of extrudate collected over a 10 minute time interval, as measured by ASTM D1238, Procedure B). The relatively high melt flow values of these virgin resins inhibit their use in many commercial applications.
One example of a commercial process which can be inhibited by using arylene sulfide resins having associated therewith high melt flow values is that wherein the resin is used to make polymeric composite material. Polymeric composite material is made by combining, with the arylene sulfide resin while in a melted phase, various reinforcing materials (e.g., fibrous reinforcements) and/or fillers. After being prepared, the polymer composite material is then often sent for further processing into a final or intermediate molded product. This latter processing step often requires that the polymer composite material be heated to a temperature above the melting point of the arylene sulfide resin used in its preparation.
The relatively high melt flow values (i.e., low melt viscosity), of virgin arylene sulfide polymers, often make it difficult to handle these resins by conventional practices. For example, when exposed to processing temperatures above their respective melting point, it becomes very difficult to process virgin arylene sulfide polymers with standard techniques and/or apparatuses. However, since arylene sulfide polymers possess many desirable properties which make them extremely useful, it would be advantageous to improve their processability without materially degrading any of their desirable properties.
One technique of improving the processability of virgin arylene sulfide resins is to oxidatively cure the virgin resins. Since oxidative curing processes typically consist of exposing a particulate virgin resin to an oxidizing atmosphere, while being heated to an elevated temperature below the resin's melting point, this process is often referred to as "solid-state" curing.
While solid-state curing a virgin arylene sulfide resin does result in decreasing the resin's melt flow value (i.e., increasing its melt viscosity), this process detrimentally affects some of the resin's mechanical and/or physical properties. Specifically, some of the detrimental effects of solid-state curing process are a substantial darkening of the resin's natural color and/or a reduction in the impact strength of material prepared from employing the resin.
Therefore, for some commercial applications, it would be desirable to have a process which decreases the melt flow value (i.e., increases the viscosity) of virgin arylene sulfide resins without implementing a solid-state curing process. Accordingly, one object of this invention is to provide a process for preparing arylene sulfide resins, having associated therewith low melt flow values, without employing a solid-state curing process.
Another technique for improving the processability of an arylene sulfide resin is demonstrated in U.S. Pat. No. 4,116,947, henceforth Patent '947. Specifically, Patent '947 discloses, among other things, methods to improve processability of arylene sulfide resins by employing, during the polymerization process, a polyhaloaromatic compound having more than two halogen substituents per molecule. This process results in branched, high molecular weight arylene sulfide resins. According to Patent '947, the gram-moles of the polyhaloaromatic compound present during polymerization, for each gram-mole of the dihaloaromatic compound present during polymerization, range from about 0.00002 to about 0.014 gram-mole, preferably, from about 0.001 to about 0.012 gram-mole. The arylene sulfide resins modified by the process disclosed in Patent '947 have significantly higher molecular weights and significantly lower melt flow values than their unmodified counterparts.
Although the polymers, made by the inventive process disclosed in Patent '947, have associated therewith many commercially desirable physical and mechanical properties, it is often desirable to produce high molecular weight arylene sulfide resins, having inherently associated therewith low melt flow values, which contain a lesser degree of branched polymeric chains. However, if this is accomplished by merely employing a lesser amount of the polyhaloaromatic compound, this will also result with forgoing some of the improvements in processability. Accordingly, another object of this invention is to provide high molecular weight arylene sulfide resins which, while having associated therewith the desired improvement in processability (i.e., low melt flow values), contain a lesser degree of branched polymeric chains.
Yet another technique for improving the processability of arylene sulfide resins is demonstrated in U.S. Pat. No. 3,919,177, henceforth Patent '177. Specifically, Patent '177 discloses, among other things, methods to improve the processability of arylene sulfide resins by employing, during the polymerization process, an alkali metal carboxylate. According to Patent '177, the gram-moles of alkali metal carboxylate present during polymerization, for each gram-mole of dihaloaromatic compound present during polymerization, ranges from about 0.05 to about 4 gram-moles, preferably, from about 0.1 to about 2 gram-moles. The arylene sulfide resins prepared by the process disclosed in Patent '177 have significantly higher molecular weights and significantly lower melt flow values in uncured form than do their uncured counterparts prepared without the use of the alkali metal carboxylate modifier of the process in Patent '177.
While polymers made by the inventive process disclosed in Patent '177 have associated therewith many commercially desirable physical and mechanical properties, polymers with properties superior to those prepared by the process therein would be even more desirable. Accordingly, yet another object of this invention is to provide high molecular weight arylene sulfide resins, prepared by the addition of at least an alkali metal carboxylate, having properties which are superior to those of resins produced in accordance with the process disclosed in Patent '177.
Although polymers made in accordance with the process disclosed in Patent '177 have associated therewith many commercial desirable properties, depending upon process employed to recover these resins from their respective polymerization reaction mixture, a processing problem can arise. Specifically, when these polymers are recovered by the process wherein the polymerization reaction mixture effluent, comprising the arylene sulfide resin, unreacted monomer, organic amide, water, and various solid and liquid by-product materials, is transported across a flash valve which rapidly reduces the pressure exerted on the reaction mixture effluent, thus resulting in separating the solidified resin from a substantial portion of the remaining liquid components (i.e., often referred to as a "flash" process), the recovered resin is generally in the form of feathery particles having a relatively low bulk density (i.e., generally less than about 15 lbs/ft.sup.3). This feathery resin material filters slowly and thus, hampers the polymer's filtering, washing and processability.
Filtering, washing and processing arylene sulfide resins, having associated therewith low bulk densities, are extremely difficult. For example, extreme difficulty is often experienced when feeding an extruder with resins having low bulk densities. Efforts to force feed extruders with an auger-fed hopper or to compact the powder with a heated two-roll mill do not overcome these difficulties. Furthermore, the fine particle size of low bulk density arylene sulfide resins introduces non-systematic errors when determining the resin's flow characteristics.
In view of the above, a process which increases the bulk density of high molecular weight arylene sulfide resins, prepared by the addition, during polymerization, of at least an alkali metal carboxylate, would result in greatly improving the filterability, washability and processability of these resins. Therefore, still another object of this invention is to provide high molecular weight arylene sulfide resins having increased bulk densities.
Other aspects, concepts and objects of this invention will become apparent from the following detailed description and appended claims.